So basically evolution is random.
No. The MUTATIONS are random. They are errors in the copying process. An insertion here, deletion there, a substitution over yonder; that sort of thing. Remember that DNA is coding for something. Whether it be protein synthesis (around 5%) or something more like gene regulation (much greater %)
A simple analogy is to think of DNA and it's A-C-G-T stuff like a line of text. You start out with a sentence...
I will not work today.
Then you add a letter in someplace (say... the letter 'r' between the 'o' and the 't' of 'not) and see what happens, making sure to keep the same structure of 1 letter, 4 letters, 3 letters, 4 letters 5 letters. This is what you get.
I will nor twor ktoda.
This is catastrophic failure and may result in big ass problems for the organism depending on where it happens. Now lets try taking a letter out and see what happens. Let's take the 'r' out of the word 'work'.
I will not wokt oday.
Less damage, but still not so great. May be good or bad for the organism. Now lets replace a letter and see what happens. Let's replace the 't' from 'not' with a 'w' and see what happens.
I will now work today.
It still makes sense but it is a wholesale change in the meaning of the sentence, or in the case of DNA, a significant change in the presentation of the individual.
So when you think about it, a simple change in the DNA pattern or sequence will give rise to a slightly different individual. The manifestation that occurs as a result of this DNA change can either give the individual an increased chance to survive in whatever environment they are in, or a decreased chance. If it gives them an increased chance to survive over others of the same species, then over long stretches of time, that new DNA strand will be more likely passed on from generation to generation, simply because the individuals with that DNA sequence are better suited to survive where they live.
Some changes help keep you alive but most will get you killed?
Actually, from what I understand, most don't do much. But the ones that are beneficial are rare, yes.
What's the most change that can occur, or has occured, in a single generation? And if it's a lot then does this occur frequently?
This is a really interesting question because you have to think of it in a few different ways. My guess is that any large mutations would almost always have catastrophic implications for any offspring, because an individual would not always be able to deal with the changes (I.E. if you were to have a genetic mutation causing you to grow a third arm, you need to have blood supply to it, nerve innervation, bone structures, etc). That would explain why we don't see superhero's running around with bullet proof skin and such. But if you want to look at survivable changes that occur within a single generation, just look up genetic disorders. Things like Cystic Fibrosis, Down Syndrome, Cri-du-chat syndrome, etc.
Here is a giant list of genetic disorders from wiki... http://en.wikipedia.org/wiki/List_of_genetic_disorders